Explain the process of sewage water treatment before it can be discharged into natural water bodies. Why is this treatment essential?
The sewage in sewage treatment plants is carried out in two ways:
(a) Primary treatment: It involves the physical removal of particles from sewage through filtration and sedimentation. Floating debris is removed by filtration and grit is removed by sedimentation. Thus, all solids which settle form the primary sludge and the supernatant forms the effluent.
Secondary treatment: The effluent from primary treatment is passed to aeration tanks where air is pumped into it. This allows the growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments) and microbes consume the major part of the organic matter in the effluent. This reduces the BOD (biological oxygen demand) of the effluent. The effluent is then passed into the settling tank where bacterial flocs are allowed to sediment. This sediment is called activated sludge. The small portion of this activated sludge is again passed to the aeration tank to serve as inocula. The remaining major part of this sludge is pumped into large anaerobic sludge digesters. Here, anaerobic bacteria digest bacteria and fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, H2S and CO2. This treatment is essential as the sewage or municipal waste discharged into
rivers, streams and other water bodies contains human excreta, organic wastes and several pathogenic microbes.
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- Microbes in Sewage Treatment